(a) Technical Field
The present disclosure relates to a fuel cell system using evaporative cooling and a method of cooling the fuel cell system, in which cooling of fuel cell stack and air humidification can be provided by a single air line passing through fuel cell stack.
(b) Background Art
A fuel cell system generates electrical energy by electrochemically converting chemical energy derived from a fuel directly into electrical energy by oxidation of the fuel.
A typical fuel cell system comprises a fuel cell stack for generating electricity by electrochemical reaction, a hydrogen supply system for supplying hydrogen as a fuel to the fuel cell stack, an oxygen (air) supply system for supplying oxygen containing air as an oxidant required for the electrochemical reaction in the fuel cell stack, a thermal management system (TMS) for removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and performing water management function, and a system controller for controlling overall operation of the fuel cell system. The fuel cell system generates heat and water as well as electricity.
One of the most attractive fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which has the highest power density among known fuel cells. The PEMFC is operated in a low temperature and is able to start up in a short time and has a fast reaction time for power conversion.
The fuel cell stack included in the PEMFC comprises a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate separator. The MEA includes a polymer electrolyte membrane through which hydrogen ions are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The bipolar plate separator functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
The fuel cell stack is consisted of a plurality of unit cells, each unit cells including an anode, a cathode and an electrolyte (electrolyte membrane). Hydrogen is supplied to the anode (“fuel electrode”) and oxygen containing air is supplied to the cathode (“air electrode” or “oxygen electrode”).
The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are transmitted to the cathode through the GDL and the bipolar plate.
At the cathode, the hydrogen ions supplied through the (polymer) electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen containing air supplied to the cathode to produce water.
Migration of the hydrogen ions cause electrons to flow through an external conducting wire, which generates electricity and heat.
The electrode reactions in the PEMFC can be represented by the following formulas:Reaction in the fuel electrode: 2H2→4H++4e−Reaction in the air electrode: O2+4H++4e−→2H2OOverall reaction: 2H2+O2→2H2O+electrical energy+heat energy
In the above reaction, the hydrogen ions permeate the polymer electrolyte membrane, and the membrane permeability of hydrogen is determined by water content of the membrane. As the above reaction proceeds, water is produced and is used to humidify the reactant gases and the membrane.
If the oxygen containing air is dried, the whole quantity of water produced by the reaction is used to humidify the oxygen containing air. As a result, the polymer electrolyte membrane is dried and the hydrogen permeability of the membrane is thus reduced. Meanwhile, if the polymer electrolyte membrane is too much wet, pores of the GDL are clogged, and thus the reactant gases are prevented from getting in contact with the catalyst. For this reason, it is very important to appropriately maintain the water content of the polymer electrolyte membrane.
Moreover, oxygen containing air supplied from atmosphere has a humidity which is not high enough to wet the membrane in an appropriate level. Thus, it is necessary to humidify the air before being supplied to the fuel cell.
In this regard, U.S. Pat. No. 5,700,595 discloses a proton exchange membrane fuel cell device in which porous plate assemblies are used to provide system cooling, reactant humidification, and condensed water collection.
FIG. 1 is a cross-sectional view of the proton exchange membrane fuel cell device disclosed in the patent, and FIG. 2 is a conceptual diagram showing how air and hydrogen are humidified by a coolant and how condensed water produced by fuel cell reaction is pumped in a fuel cell stack employing the porous plate assemblies.
Referring to FIGS. 1 and 2, air and hydrogen are humidified by a coolant flowing through coolant channels 32 and 32′, and condensed water generated by a fuel cell reaction is absorbed (collected) by the coolant due to vacuum pressure.
In more detail, the coolant channels 32 and 32′ are provided in the middle of porous plates 26 and 26′, and air channels 29 and 29′ and hydrogen channels 30 and 30′ are provided on both sides thereof such that the coolant is circulated through the coolant channels 32 and 32′, and the (oxygen containing) air and hydrogen are supplied through the air channels 29 and 29′ and the hydrogen channels 30 and 30′, respectively.
At this time, the air and hydrogen are supplied at a pressure higher than that of the circulating coolant such that the water generated at the air electrode is pumped to the coolant channels 32 and 32′ through the porous plates 26 and 26′.
Since the porous plates 26 and 26′ are saturated with the circulating coolant, they serve to humidify the oxidant reactant (air) and fuel reactant (hydrogen), and thus the coolant can perform the operations of cooling and humidifying the system, and removing product water.
However, the fuel cell water management system has the following drawbacks. Since the coolant is condensed and humidified repeatedly, it is necessary to use deionized pure water only; an antifreezing solution cannot be used as the coolant. As the pure water is frozen at a temperature below the freezing point, the water passing through the porous plates is frozen at the air electrode or hydrogen electrode to clog the channels and has thus to be drawn out of the system after operation. The deionized pure water may also be frozen in a water tank and a lot of energy has thus to be used to thaw the frozen water during cold start-up, increasing the start-up time. Furthermore, it is necessary to provide a design that can solve the freezing problem of all components/parts related to the pure water.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.